The present invention relates to rolls for use particularly in nipped roll presses, in which rolls exert pressing forces on webs for forming paper, textile material, plastic foil and other related materials. Although the present invention may be used in the above industries, the discussion to follow will focus on the function of rolls particularly in the manufacture of paper.
The first stage of a papermaking process requires deposition of headbox stock on a forming fabric. As the stock is deposited, a great deal of the white water therein flows through the interstiches of the fabric. That which is left thereon, a combination of water and fibers, travels on the fabric and goes through one or more dewatering stages so that a resulting fibrous web or matt is left thereon. One such dewatering stage is effected by passing the web through rolls forming a nip press or series thereof, during which process water is expelled from the web. A problem common to such rolls forming the nip press, is the lack of uniformity in the pressure distributed therealong. Such lack of uniformity often results in paper of poor quality.
Conventional rolls for use in a press section may be formed of one or more layers of material. Roll deflection, commonly due to sag or nip loading, has been a problem in the industry. Rolls have been developed which monitor and alter the crown to compensate for deflection. These rolls have a floating shell which surrounds a stationary core. Underneath the floating shell are pressure regulators which detect pressure differentials and compensate therefor.
One such roll is described in U.S. Pat. No. 4,509,237. This roll has position sensors to determine an uneven disposition of the roll shell. The signals from the sensors activate support or pressure elements underneath the roll shell, to equalize any uneven positioning that may exist due to pressure variations. The pressure elements comprise conventional hydrostatic support bearings which are supplied by a pressurized oil infeed line. A similar roll is disclosed in U.S. Pat. No. 4,729,153. This controlled deflection roll further has sensors for regulating roll surface temperature in a narrow band across the roll face. Other controlled deflection rolls such as the one described in U.S. Pat. No. 4,233,011, rely on the thermal expansion properties of the roll material, to achieve proper roll flexure. Such deflection compensated rolls are effective in varying the crown. Thus, such rolls can operate as effectively at a loading of 100 pounds per inch as at 500 pounds per inch, whereas rolls without such capabilities can only operate correctly at a single specific loading.
However, a problem inherent to both controlled deflection rolls and plain rolls is that there is no way to measure the loading across the roll face while the roll is in operation. If roll loading in a controlled deflection roll is set to 200 pounds/inch, it may actually be 300 pounds/inch at the edges and 100 pounds/inch in the center.
Conventional methods of determining the presence of such discrepancies in applied pressure require stopping the roll and placing a long piece of carbon paper or pressure sensitive film in the nip. This procedure is known as taking a nip impression. This procedure, although useful, cannot be used while the nip press is in operation. Moreover, temperature and other related changes which would affect the uniformity of nip pressure, cannot be taken into account.
The roll described in U.S. Pat. No. 4,898,012 has attempted to address this problem by incorporating sensors on the roll to determine the pressure profile of a press nip. However, there are a number of problems inherent to this roll. Primarily, the construction requires a stationary center beam, and as such would not be adapted to all types of rolls, but only controlled deflection rolls having a floating roll shell. Additionally, the sensors are discrete sensors which yield only a small number of readings. Furthermore, each sensor must be connected separately, leading to wiring and connection problems. Additionally, the spaced locations of the sensors may lead to inaccuracies as they may fail to detect a portion of the material where an uneven profile exists. Furthermore, the system requires the positioning of two nips 180.degree. apart, so as to guarantee the ability to sense an out-of-round condition. The positioning of nips in such a manner is customary in steel mills or plate rolling, but is seldom so positioned in a papermaking process.